The invention relates generally to power transmission and more specifically to a system and method for transmitting electrical power to sub-sea electrical equipment.
Sub-sea electrical equipment such as a sub-sea motor which drives a gas compressor has a higher nominal power rating (e. g., in the order of 10 or 15 MW). As such, sub-sea compression clusters may be required to transmit a total power in the order of 50 to 100 MW over a distance of 100 or 200 km. The transmission of high power over a distance of more than 100 km and distributing the power sub-sea is a very challenging problem. Such a transmission is done at high voltages to reduce losses. At the receiving sub-sea end, the voltage is stepped-down and then distributed to the individual loads. Distribution distances are typically much shorter than the transmission distance.
Three phase alternating current (AC) power transmission and distribution is one way of transmitting power to subsea equipment. AC power transmission, although mature, provides technical challenges for applications where bulk power is transmitted over long cables. Due to cable capacitance, a significant amount of reactive power needs to be provided by the power source and carried by the cable. Capacitance causes charging current to flow along the length of the AC cable. Because the cable must carry this charging current in addition to the useful load current, the cable losses are high; the cables are over-rated and expensive. Large reactive power requirements may trigger power system stability issues.
The limitation of AC transmission and distribution can be alleviated by direct current (DC) transmission. High voltage (HV) DC transmission typically requires the usage of power electronic converters in the transmission systems that are capable of converting between HVAC and HVDC. Line commutated converters (LCC) and voltage sourced converters (VSC) are examples of such power electronic converters. However, LCC converters require significant amount of filters to supply for desired reactive power whereas VSC converters require large DC capacitors which affects reliability and maintenance.
Therefore, there still exists a need for a compact and a reliable system and method for transmitting electric power to subsea equipment.